1. Field of the Invention
The present invention relates to an apparatus for controlling a flow resistance of a photoresist solution, and more particularly, to an apparatus with a check function for controlling a flow resistance of a photoresist solution.
2. Description of the Prior Art
In a lithographic process, electrical patterns are formed by performing exposure and development processes to a photoresist that has been coated onto the surface of a semiconductor wafer. The photoresist solution is drawn from a storage tank, transported via pipes to a nozzle and then sprayed onto the surface of the semiconductor wafer during the coating process. The viscosity of the photoresist solution varies from one storage tank to another as photoresist solutions of different viscosities are required for different manufacturing conditions. The change of viscosity affects the flow rate of the photoresist solution. These changing flow rates of the photoresist solution cause disproportionate coatings of photoresist onto the surface of the semiconductor wafer and adversely affect the yield of the subsequent fabrication process. Hence, it is important to control the flow resistance of the photoresist solution so as to transport the photoresist solution with a substantially constant pressure.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a photoresist solution supply device 10 according to the prior art. The photoresist solution supply device 10 comprises a tank 12 for storing the photoresist solution, a pipe 14 partially submerged in the photoresist solution in the tank 12 to transport the photoresist solution, a buffer tank 16 connected to the pipe 14 to remove bubbles from the photoresist solution, a pump 24 connected to the buffer tank 16 to draw the photoresist solution from the buffer tank 16, a nozzle 34, a bellow 26 connected to the pump 24, and an air cylinder 28 connected with the bellow 26. The pump 24 has compression and extension cycles, and pumps photoresist to the nozzle 34 on the compression cycle. The bellow 26 is air-powered and delivers the mechanical motion required to drive the pump 24. The air cylinder 28 comprises two valves 30, 32 and the to-and-fro movement of the bellow 26 is generated by compressed air flowing into the air cylinder 28 from the different valves 30, 32.
The buffer tank 16 comprises an inlet 18 on its top that is connected to the pipe 14, an outlet 22 on its bottom, and a vent 20 set at the top of the buffer tank 16 to vent bubbles from the photoresist solution. When the photoresist solution is introduced into the buffer tank 16, the bubbles in the photoresist solution accumulate in the top of the buffer tank and are vented through the vent 20 to prevent bubbles from interfering with the photoresist coating on the surface of the semiconductor.
To move the photoresist solution from the tank 12 to the nozzle 34, compressed air flows into the air cylinder 28 from the valve 32. This causes the bellow 26 to pull on the pump 24, which extends the pump 24 and causes it to draw photoresist from the tank 12. Then, compressed air flows into the air cylinder 28 from the valve 30. This causes the bellow 26 to push on the pump 24, compressing it and forcing the photoresist solution from the pump 26 to the nozzle 34. The buffer tank 16 draws photoresist solution from the tank 12 through the pipe 14 to replace the photoresist solution drawn by the pump 24. When the tank 12 runs out of photoresist solution, the pipe 14 is extracted from the tank 12 and inserted into a new tank. During this change, the photoresist solution in the pipe 14 drains out and air flows in. When the pipe 14 is inserted into the new tank, bubbles form in the photoresist solution. These bubbles are coated onto the surface of the semiconductor wafer together with the photoresist solution, degrading the quality of the photoresist coating process.
The viscosity of the photoresist solution can vary from one tank to another as photoresist solutions with differing viscosities are required by various manufacturing conditions. The change of viscosity affects the volume of photoresist solution transported into the pump 24, and may also cause bubbles to form in the photoresist solution. If the viscosity of the photoresist solution in the new tank is less than that of the previous tank, and the pump 24 draws the photoresist solution at the same speed, the volume of the photoresist solution drawn into the pump 24 will increase, leading to waste of the photoresist solution. If the flow speed of the photoresist solution drawn into the pump 24 is too great, bubbles will form in the photoresist solution. Therefore, the driving pressure of air forced into the air cylinder 28 must be changed to prevent the formation of bubbles in the photoresist solution. The optimum driving pressure, however, is arrived at through trial and error. The transport of photoresist solution to the nozzle 34 may not go smoothly during the trial and error process. This leads to uneven coatings of photoresist and decreases the quality of the coating process.
It is therefore a primary objective of the present invention to provide a flow-control apparatus with a check function for controlling the flow resistance of a photoresist solution.
In a preferred embodiment, the present invention provides a flow-control apparatus with a check function for controlling the flow resistance of a photoresist solution. The apparatus is connected to a photoresist supply device, the photoresist supply device comprises a tank for storing the photoresist solution, a pipe partially submerged in the photoresist solution in the tank to transport the photoresist solution, and a pump for drawing the photoresist solution out of the tank, the apparatus comprising:
a housing comprising a chamber, a top opening positioned at the top of the chamber, and a bottom opening positioned at the bottom of the chamber; wherein the top opening can be mated to an end of the pipe or to a bottom opening of another housing;
a sphere moveably set inside the chamber of the housing, the sphere being used to increase the flow resistance of the photoresist solution;
wherein the number of the apparatuses mounted in series with the end of the pipe can be changed to control the flow resistance of the photoresist solution through the pipe when drawing the photoresist solution from the tank.
It is an advantage of the present invention that the present invention can prevent back-flow of the photoresist solution into the tank so that air cannot flow into the pipe during the changing of one tank for another. The present invention controls the flow resistance of the photoresist solution by changing the number of apparatuses connected to the pipe, the density of the sphere, the number of spheres in the housing and the size of the sphere. A checklist can be made that relates the viscosity of the photoresist solution with the number of apparatuses, the density of the sphere, the number of spheres and the size of the sphere so that the flow resistance of the photoresist solution through the pipe can be easily controlled, thereby increasing the quality of the coating process.